What needs syncing?

├─ Structured data (records, relationships)?
│  ├─ Using SwiftData? → SwiftData + CloudKit (easiest, iOS 17+)
│  ├─ Need shared/public database? → CKSyncEngine or raw CloudKit
│  └─ Custom persistence (GRDB, SQLite)? → CKSyncEngine (iOS 17+)
│
├─ Documents/files users expect in Files app?
│  └─ iCloud Drive (UIDocument or FileManager)
│
├─ Large binary blobs (images, videos)?
│  ├─ Associated with structured data? → CKAsset in CloudKit
│  └─ Standalone files? → iCloud Drive
│
└─ App settings/preferences?
   └─ NSUbiquitousKeyValueStore (simple key-value, 1MB limit)

// ✅ CORRECT: Offline-first architecture
class OfflineFirstSync {
    private let localStore: LocalDatabase  // GRDB, SwiftData, Core Data
    private let syncEngine: CKSyncEngine

    // Write to LOCAL first, sync to cloud in background
    func save(_ item: Item) async throws {
        // 1. Save locally (instant)
        try await localStore.save(item)

        // 2. Queue for sync (non-blocking)
        syncEngine.state.add(pendingRecordZoneChanges: [
            .saveRecord(item.recordID)
        ])
    }

    // Read from LOCAL (instant)
    func fetch() async throws -> [Item] {
        return try await localStore.fetchAll()
    }
}

// ❌ WRONG: Cloud-first (blocks on network)
func save(_ item: Item) async throws {
    // Fails when offline, slow on bad network
    try await cloudKit.save(item)
    try await localStore.save(item)
}

// Server always has latest, client accepts it
func resolveConflict(local: CKRecord, server: CKRecord) -> CKRecord {
    return server  // Accept server version
}

// Combine changes from both versions
func resolveConflict(local: CKRecord, server: CKRecord) -> CKRecord {
    let merged = server.copy() as! CKRecord

    // For each field, apply custom merge logic
    merged["notes"] = mergeText(
        local["notes"] as? String,
        server["notes"] as? String
    )
    merged["tags"] = mergeSets(
        local["tags"] as? [String] ?? [],
        server["tags"] as? [String] ?? []
    )

    return merged
}

// Present conflict to user
func resolveConflict(local: CKRecord, server: CKRecord) async -> CKRecord {
    let choice = await presentConflictUI(local: local, server: server)
    return choice == .keepLocal ? local : server
}

import SwiftData

// Automatic CloudKit sync with zero configuration
@Model
class Note {
    var title: String
    var content: String
    var createdAt: Date

    init(title: String, content: String) {
        self.title = title
        self.content = content
        self.createdAt = Date()
    }
}

// Container automatically syncs if CloudKit entitlement present
let container = try ModelContainer(for: Note.self)

// For GRDB, SQLite, or custom databases
class MySyncManager: CKSyncEngineDelegate {
    private let engine: CKSyncEngine
    private let database: GRDBDatabase

    func handleEvent(_ event: CKSyncEngine.Event) async {
        switch event {
        case .stateUpdate(let update):
            // Persist sync state
            await saveSyncState(update.stateSerialization)

        case .fetchedDatabaseChanges(let changes):
            // Apply changes to local DB
            for zone in changes.modifications {
                await handleZoneChanges(zone)
            }

        case .sentRecordZoneChanges(let sent):
            // Mark records as synced
            for saved in sent.savedRecords {
                await markSynced(saved.recordID)
            }
        }
    }
}

import UIKit

class MyDocument: UIDocument {
    var content: Data?

    override func contents(forType typeName: String) throws -> Any {
        return content ?? Data()
    }

    override func load(fromContents contents: Any, ofType typeName: String?) throws {
        content = contents as? Data
    }
}

// Save to iCloud Drive (visible in Files app)
let url = FileManager.default.url(forUbiquityContainerIdentifier: nil)?
    .appendingPathComponent("Documents")
    .appendingPathComponent("MyFile.txt")

let doc = MyDocument(fileURL: url!)
doc.content = "Hello".data(using: .utf8)
doc.save(to: url!, for: .forCreating)

// ❌ WRONG: No awareness of pending changes
var items: [Item] = []  // Are these synced? Pending? Conflicted?

// ✅ CORRECT: Track sync state
struct SyncableItem {
    let item: Item
    let syncState: SyncState  // .synced, .pending, .conflict
}

// ❌ WRONG: UI blocks until sync completes
func viewDidLoad() async {
    items = try await cloudKit.fetchAll()  // Spinner forever on airplane
    tableView.reloadData()
}

// ✅ CORRECT: Show local data immediately
func viewDidLoad() {
    items = localStore.fetchAll()  // Instant
    tableView.reloadData()

    Task {
        await syncEngine.fetchChanges()  // Background update
    }
}

// ❌ WRONG: Single attempt
try await cloudKit.save(record)

// ✅ CORRECT: Exponential backoff
func saveWithRetry(_ record: CKRecord, attempts: Int = 3) async throws {
    for attempt in 0..<attempts {
        do {
            try await cloudKit.save(record)
            return
        } catch let error as CKError where error.isRetryable {
            let delay = pow(2.0, Double(attempt))
            try await Task.sleep(for: .seconds(delay))
        }
    }
    throw SyncError.maxRetriesExceeded
}

enum SyncState {
    case synced       // ✓ (checkmark)
    case pending      // ↻ (arrows)
    case conflict     // ⚠ (warning)
    case offline      // ☁ with X
}

// In SwiftUI
HStack {
    Text(item.title)
    Spacer()
    SyncIndicator(state: item.syncState)
}